Summary:
Customer docs moved to github repo and submodule added.
This will allow contractors to contribute the github repo.

Test Plan: N/A

Reviewers: michelle, #engineering

Reviewed By: michelle, #engineering

Differential Revision: https://phab.corp.pixielabs.ai/D3472

GitOrigin-RevId: 68b02bc160c803b0f434c05515bccb2eb14ecdaf